The present invention relates generally to longitudinally expanded microporous tubular polytetrafluoroethylene grafts, and more particularly, to radially expandable polytetrafluoroethylene ("rePTFE") grafts which are longitudinally expanded and sintered prior to radial expansion. The radially expandable polytetrafluoroethylene grafts of the present invention are particularly well suited for covering an endoluminal prosthesis, being endoluminally delivered to a site within a mammalian body, and radially expanded in vivo to restore an anatomical passageway or to create a passageway.
Microporous expanded polytetrafluoroethylene ("ePTFE") tubes may made by any one of different, but well known, methods. Expanded polytetrafluoroethylene is typically made by admixing particulate dry polytetrafluoroethylene resin with a lubricant to form a viscous slurry. The admixture is poured into a mold, typically a cylindrical mold, and compressed under the influence of a positive pressure to form a cylindrical billet. The billet is then ram extruded through an extrusion die into either tubular or sheet structures termed in the art as extrudates. The extrudates consist of extruded polytetrafluoroethylene-lubricant admixture, termed in the art as "wet PTFE." Wet PTFE has a microstructure of coalesced, coherent PTFE resin particles in a highly crystalline state. After extrusion, the wet PTFE is exposed to a temperature below the flash point of the lubricant to volatilize a major fraction of the lubricant from the PTFE extrudate. The resulting PTFE extrudate without a major fraction of lubricant is known in the art as dried PTFE. The dried PTFE is then either uniaxially, biaxially or radially expanded using appropriate mechanical apparatus known in the art. Expansion is typically carried out at an elevated temperature, e.g., above room temperature but below 327.degree. C., the crystalline melt point of polytetrafluoroethylene. Uniaxial, biaxial or radial expansion of the dried PTFE causes the coalesced, coherent PTFE resin to form fibrils emanating from nodes, with the fibrils oriented parallel to the axis of expansion. Once expanded, the dried PTFE is referred to as expanded PTFE ("ePTFE") or microporous PTFE. The ePTFE is then transferred to a heating oven and heated to a temperature above 327.degree. C., the crystalline melt point of PTFE, while restraining the ePTFE against uniaxial, biaxial radial contraction, to sinter the ePTFE, thereby causing at least a portion of the crystalline PTFE to undergo a physical change from a crystalline structure to an amorphous structure. The conversion from a highly crystalline structure to an increased amorphous content, which results from sintering, serves to lock the node and fibril microstructure, as well as its orientation relative to the axis of expansion, and provides a dimensionally stable tubular or sheet material upon cooling. Expansion may also be carried out at a temperature below the vapor point of the lubricant. However, prior to the sintering step, the PTFE must be dried of lubricant because the sintering temperature of PTFE is greater than the flash point of commercially available lubricants.
Sintered ePTFE articles exhibit significant resistance to further uniaxial, or radial expansion. This property has led many in the art to devise techniques which entail endoluminal delivery and placement of an ePTFE graft having a desired fixed diameter, followed by endoluminal delivery and placement of an endoluminal prosthesis, such as a stent or other fixation device, to frictionally engage the endoluminal prosthesis within the lumen of the anatomical passageway. The Kreamer patent, U.S. Pat. No. 5,078,726, issued in 1992, exemplifies such use of an ePTFE prosthetic graft. Kreamer discloses a method of excluding an abdominal aortic aneurysm which entails providing a tubular PTFE graft which has a diameter corresponding to that of the inside diameter of a healthy section of the abdominal aorta, delivering the tubular PTFE graft and positioning the graft so that it spans the abdominal aorta. Prosthetic balloon expandable stents are then delivered and placed proximal and distal the abdominal aorta and within the lumen of the tubular PTFE graft. The prosthetic stents are then balloon expanded to frictionally engage the proximal and distal ends of the tubular PTFE graft against the inner luminal wall of healthy sections of the abdominal aorta.
Similarly, published International Applications No. WO95/05132 and WO95/05555, both published Feb. 23, 1995, filed by W. L. Gore Associates, Inc., disclose balloon expandable prosthetic stents which have been covered on inner and outer surfaces of the stent by wrapping ePTFE sheet material about the balloon expandable prosthetic stent in its enlarged diameter, sintering the wrapped ePTFE sheet material to secure it about the stent, then the assembly is crimped down to a reduced diameter for endoluminal delivery using a balloon catheter. Once positioned endoluminally, the stent-graft combination is then dilatated to re-expand the stent to its enlarged diameter and return the ePTFE wrapping to its original diameter. Thus, the original unexpanded diameter of the ePTFE wrap delimits diametric expansion of the stent and the ePTFE wrap is returned to its original uncrimped diameter.
Thus, it is well known in the prior art to provide an ePTFE covering which is fabricated at the final desired endovascular diameter and is endoluminally delivered in a folded or crimped condition to reduce its delivery profile, then unfolded in vivo using either the spring tension of a self-expanding, thermally induced, expanding structural support member or a balloon catheter.
In contradistinction to the prior art, the present invention provides a radially, plastically deformable tubular ePTFE material, having a microstructure of nodes interconnected by fibrils, with the nodes being substantially perpendicular to the longitudinal axis of the tubular ePTFE material and the fibrils being oriented parallel to the longitudinal axis of the tubular ePTFE material. Radial expansion of the inventive ePTFE material deforms the ePTFE microstructure by elongating the nodes while substantially retaining the internodal distances (IND) between adjacent nodes in the longitudinal axis of the ePTFE tube.
As used herein, the following terms have the intended meanings as indicated.
"Fibril" refers to a strand of PTFE material which originates from one or more nodes and terminates at one or more nodes.
"Internodal Distance" or "IND" refers to an average distance between two adjacent nodes measured along the longitudinal axis of each node between the facing surfaces of the adjacent nodes. IND is expressed in microns (.mu.) as the unit of measure.
"Node" refers to the solid region within an ePTFE material at which fibrils originate and converge.
"Node Length" as used herein refers to a distance measured along a straight line between the furthermost opposing end points of a single node.
"Nodal Elongation" as used herein refers to expansion of PTFE nodes in the ePTFE microstructure along the longitudinal axis of a node, or along the Node Length.
"Node Width" as used herein refers to a distance measured along a straight line drawn perpendicular to the longitudinal axis of a node between opposing longitudinal surfaces of a node.
"Plastic Deformation" as used herein refers to the radial deformation of the ePTFE microstructure under the influence of a radially expansive force which deforms and elongates the Node Length and results in elastic recoil of the ePTFE material less than about 25%.
"Radially Expandable" as used herein to describe the present invention refers to a property of the ePTFE tubular member to undergo radially-oriented Plastic Deformation mediated by Nodal Elongation.
"Structural Integrity" as used herein to describe the present invention refers to a condition of the ePTFE microstructure both pre and post-radial deformation in which the fibrils are substantially free of fractures or breaks and the ePTFE material is free of gross failures.
The inventive ePTFE material of the present invention is capable of being radially expanded under the influence of a radially outward force applied from the lumen of the ePTFE tubular material to substantially uniformly deform the ePTFE material. The inventive ePTFE material is radially expandable to a diameter 700% its unexpanded diameter under the influence of pressures less than 6 atm, preferably less than or equal to about 4.0 to 4.5 atm., most preferably between 2-3 atm., while retaining the structural integrity of the ePTFE microstructure. Conservation of the structural integrity of the ePTFE material is determined by conservation of the ePTFE microstructure structural integrity. During and after radial expansion up to and including about 700% of the original unexpanded diameter, the ePTFE microstructure structural integrity is considered conserved where the following factors are met: 1) IND remains substantially the same as the unexpanded graft; 2) water entry pressure as measured by Association for the Advancement of Medical Instrumentation (AAMI) test method 8.2.4 remains within .+-.60% of the water entry pressure of the unexpanded graft; 3) the wall thickness of the graft, as determined by AAMI test method 8.7.4, maintains its concentricity as determined by a substantially uniform wall thickness within .+-.30% about the circumference of the graft; 4) average post-radial expansion wall thickness remains within about .+-.70% of the average pre-radial expansion wall thickness as determined by AAMI test method 8.7.4; 5) longitudinal tensile strength as measured by AAMI test method 8.3.2 remains within .+-.100% of the value of the unexpanded graft, when normalized for wall thickness; 6) radial tensile strength as measured by AAMI test method 8.3.1 remains within .+-.40% of the value of the unexpanded graft, when normalized for wall thickness; and 7) is free of gross tears or fractures.